It is known to provide a regulated output voltage from a power supply, the power supply including a closed loop feedback path to maintain its regulated output voltage at a desired level, referred to as a set-point. For example, a predetermined fraction of the output voltage of the power supply may be compared with a reference voltage, and the result of the comparison may be used to control a PWM (pulse width modulation) function of a DC-DC converter or a control element of a linear voltage regulator.
It is also known to provide a power supply with a trim input, to which a control signal can be supplied to trim the output voltage of the power supply. For example, such trimming can be used to adjust the output voltage to a desired value, to compensate for changes of the power supply characteristics due to temperature changes or aging, or for margining, in which for example the output voltage of the power supply is raised or lowered from the set-point to test operation of a supplied electronic circuit at supply voltages above and below its nominal supply voltage. This is referred to as active trim or active DC output control of the power supply.
Active trim can be carried out in an open loop or a closed loop manner. In open loop active trim, the trim input of the power supply is controlled in a desired manner without this control being dependent upon monitoring of the power supply output voltage. With closed loop active trim, the trim input of the power supply is controlled in a manner dependent upon, for example, the monitored output voltage of the power supply.
Consequently, with closed loop active trim there are two closed loops which control the power supply. Stable operation of the power supply requires that the closed loop active trim not interfere with the power supply voltage regulating closed loop; this may be difficult to ensure under all operating conditions, including for example initial start-up conditions and voltage changes for margining.
It is known for example from Summit Microelectronics, Inc. Application Note 39 entitled “SMM105/205/605/665 ADOC Tutorial and Performance Summary” dated Jun. 19, 2003, and from an article by Tom DeLurio and George Hall entitled “How you can manage multiple voltages in portables”, Power Management DesignLine newsletter, Mar. 23, 2005, to provide closed loop active DC output control of a power supply, using significant filtering and signal conditioning of the power supply output voltage to produce a voltage which is compared with a reference voltage to control charging of a capacitor. The control signal for the trim input of the power supply is dependent upon the voltage to which the capacitor is charged.
To reduce the risks of instability, in such a known arrangement the active DC output control loop has a response that is much slower than the response of the power supply voltage regulating closed loop, and in addition uses a nonlinear digital control element where adjustments of the output are always of the same magnitude. However, this introduces a ripple voltage on the power supply output voltage. Although, as described in Summit Microelectronics, Inc. Application Note 37 entitled “SMM665, 605, 205, 105 Supply Voltage Marginers and Active DC Output Controllers Component Selection for Optimum Performance” dated Jun. 18, 2003, the magnitude of the ripple voltage can be reduced by increasing the capacitance of the capacitor, this increases the margin settling time.
In such power supply control arrangements, parameters such as the nominal voltage, minimum and maximum voltages for margining, and control parameters may be stored in an EEPROM or other non-volatile memory and may be downloaded into shadow registers of a control unit on power-up. The control unit may be provided for controlling a plurality of power supplies, in which case it requires such shadow registers for each of the power supplies that it can control. As the shadow registers require a significant proportion of the total area of an integrated circuit implementation of the control unit, it is desirable to reduce as much as possible the size of (number of bits stored by) the shadow registers, without detracting from the operation or accuracy of the control unit.
Accordingly, it is desirable to provide improvements in power supply output voltage trimming and in a control unit for such trimming.